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Carb101: Impulse Chamber

In the last post we took a slightly closer look at how the diaphragm carburettor works - now we're going in even closer, with a look at the impulse chamber, straight after the jump...

We have seen that the diaphragm is used to pump fuel from the tank, in to the carb, and that it's controlled by pressure pulses from the crankcase. In this post we'll take a closer look at the impulse chamber.

The bit we're interested in can be seen on the photo on the right - that small brass pipe is connected to the crankcase at one end, and feeds in to the impulse chamber at the other end. Removing this cover (a single screw) reveals a gasket and the actual impulse diaphragm - I'm not exactly sure what material the diaphragm is made of, but it looks a little like mica.

The diaphragm has a couple of small flaps cut in to it, these cover two small holes that lead in to / from a small chamber. Those flaps are controlled by the pressure pulses being sent down that brass pipe.

On the left, we can see the actual diaphragm, with the two flaps - one just above the central screw thread, and the other slightly down and to the left of the central screw thread. The flap at lower left covers the input from the fuel line (linked to the fuel tank). On the right hand side of the photo, you can see a chamber underneath the diaphragm - consider this as a little reservoir for the moment. The flap above the central screw thread is the one controlled by pressure pulses (we'll call it the impulse flap) - as these pulses are received, the flap will open and close, the effect of which is to draw fuel from the fuel line, through the first flap and down the small hole that you can clearly see next to this first flap. The fuel is then held in that small reservoir area, until it gets pumped up through the impulse flap and down the small hole next to it. You can just make out a large circle under the diaphragm at this point...

...which has a very fine gauze filter in it - as you can see in the photo on the right; which shows the area with the diaphragm removed. This is actually the top end of a hole drilled straight through the carburettor, leading to the metering chamber on the other side; in fact at the other end of that hole is actually a needle controlled by another diaphragm.

So far then, we've followed the fuel being pumped from the fuel line, through the flaps cut in to the diaphragm and through a fine gauze filter to be delivered to the metering chamber. In the next post we'll continue to follow the fuel's journey through this second chamber and out through the main jet.